TY - JOUR

T1 - Transition enthalpies and entropies of high pressure zinc metasilicates and zinc metagermanates

AU - Leinenweber, Kurt

AU - Navrotsky, Alexandra

AU - McMillan, Paul

AU - Ito, Eiji

PY - 1989/11

Y1 - 1989/11

N2 - The enthalpies of transition at T= 298 K between zinc metasilicate assemblages, measured by molten oxide solution calorimetry, are: {Mathematical expression}(1) {Mathematical expression}(2) For reaction (1), the measured enthalpy is lower than the value 15.7±2.0 kJ/mol from the phase boundary of Syono et al. The P-T slope derived from the calorimetric data has a somewhat greater value than that of the line chosen by Syono et al., but is still consistent with the individual data points in that study. For reaction (2), the measured enthalpy of transformation is in close agreement with the value 54.0±5.1 kJ/mol estimated from the phase boundary of Akimoto et al, and with the value 50.9±3.2 kJ/mol from the phase boundary of Ohtani, which is parallel to and 0.6 GPa lower than Akimoto's. All estimates assume a pressure-independent volume of transition. For the zinc metagermanate composition, measured heats of transformation are: {Mathematical expression}(3) {Mathematical expression}(4) Raman and infrared spectra of ZnSiO3 clinopyroxene and ilmenite are presented. The entropy change for the clinopyroxene → ilmenite transiton is calculated from the spectra and estimates for the directionally-averaged acoustic velocities, using the Kieffer model for the vibrational density of states. The value ΔS = -12±5 J/mol·K, calculated using a variety of plausible densities of states, agrees with the value from phase equilibria, ΔS = -12.7±6.0 J/mol·K.

AB - The enthalpies of transition at T= 298 K between zinc metasilicate assemblages, measured by molten oxide solution calorimetry, are: {Mathematical expression}(1) {Mathematical expression}(2) For reaction (1), the measured enthalpy is lower than the value 15.7±2.0 kJ/mol from the phase boundary of Syono et al. The P-T slope derived from the calorimetric data has a somewhat greater value than that of the line chosen by Syono et al., but is still consistent with the individual data points in that study. For reaction (2), the measured enthalpy of transformation is in close agreement with the value 54.0±5.1 kJ/mol estimated from the phase boundary of Akimoto et al, and with the value 50.9±3.2 kJ/mol from the phase boundary of Ohtani, which is parallel to and 0.6 GPa lower than Akimoto's. All estimates assume a pressure-independent volume of transition. For the zinc metagermanate composition, measured heats of transformation are: {Mathematical expression}(3) {Mathematical expression}(4) Raman and infrared spectra of ZnSiO3 clinopyroxene and ilmenite are presented. The entropy change for the clinopyroxene → ilmenite transiton is calculated from the spectra and estimates for the directionally-averaged acoustic velocities, using the Kieffer model for the vibrational density of states. The value ΔS = -12±5 J/mol·K, calculated using a variety of plausible densities of states, agrees with the value from phase equilibria, ΔS = -12.7±6.0 J/mol·K.

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U2 - 10.1007/BF00209704

DO - 10.1007/BF00209704

M3 - Article

AN - SCOPUS:0542426733

SN - 0342-1791

VL - 16

SP - 799

EP - 808

JO - Physics and Chemistry of Minerals

JF - Physics and Chemistry of Minerals

IS - 8

ER -